Synthesis and characterization of a chiral spirobifluorene cyclic hexamer

Abstract

A new chiral macrocycle 1-[6], consisting of six chiral spirobifluorene units linked in a cyclic arrangement, was successfully synthesized via a homo-coupling of the corresponding acyclic trimer. Notably, this chiral cyclic hexamer exhibited flexibility in solution, and both low-temperature NMR experiments and DFT calculations suggest that its stable structure is not the hexagonal D6-symmetric structure but rather the figure-eight shaped D2-symmetric structure. In this D2-symmetric structure, the opposing bifluorenyl units are π-stacked, and it is suggested that this π-stacked region undergoes rapid dissociation and reformation at room temperature. The HOMO is distributed over the π-stacked region, with a HOMO-LUMO gap of 4.02 eV. The macrocycle exhibited strong violet fluorescence. The absorption dissymmetry factor |gabs| was 7.4 x 10-3, which is larger compared to the series of chiral smaller macrocycles 1-[n] (n = 3-5). Additionally, the CPL efficiency, indicated by a BCPL value of 189 M-1 cm-1, is relatively high among chiral organic luminescent molecules.